Concrete arch structure and method of constructing the same



Jan. 18, 1955 c, MQLKE 2,699,739

CONCRETE ARCH STRUCTURE AND METHOD OF CONSTRUCTING THE SAME Filed Aug. 5, 1950 2 Shee'ts-Sheet 1 IN V EN TOR. ER 10 C. MOLKE.

Jan. 18, 1955 E. c. MOLKE 2,699,739

CONCRETE ARCH STRUCTURE AND METHOD OF CONSTRUCTING THE SAME Filed Aug. 5, 1950 2 Sheets-Sheet 2 4 L 1 m B H 7 l? VIII, 6

IN V EN TOR. fR/c C. Moms.

QT-r-ORNE United States Patent" CONCRETE ARCH STRUCTURE AND METHOD OF CONSTRUCTING THE SAME Eric C. Molke, Highland Park, Ill.

Application August 5, 1950, Serial No. 177,954

21 Claims. (Cl. 108-1) My invention relates to a unique composite arch structure and to a new method of constructing such structures. More specifically, the invention pertains to the use of poststressed concrete elements in combination with a skeleton arch structure in a manner not known or practiced heretofore, and which results in a structure having far greater strength, stability and rigidity and, at the same time, one which is more economical to erect and in the use of materials.

The application of prestressing principles to concrete and masonry structures is well known for the purpose of removing limiting factors in conventional designs. The principal one of these factors is of course, the notorious weakness of concrete in tension. As a consequence, concrete has very poor characteristics in resisting shearing stresses and tendencies to buckle. By properly prestressing the structure, however, a homogeneous body is obtained in which the concrete acts only in compression, and the steel acts only in tension. It follows that the neutral axis can be controlled to lie entirely outside the section. Like results can be obtained by poststressing the concrete, i. e., by exteriorly applying a compression load on the portion of the concrete which would otherwise be active in tension, and imposing this load after the concrete has set and aged. Prestressing, on the other hand, involves tensioning the steel reinforcing before the concrete sets, and releasing the tension after a bond is obtained to compress the concrete. Hence, in general, prestressing and poststressing indicate whether the concrete is compressed by pretensioned steel reinforcement bonded thereto, or by posttensioned steel which is not bonded to the concrete at all, or is bonded thereto afterward.

Although prestressing techniques have many advantages over non-stressed reinforcing, it is nevertheless attended by certain disadvantages. In the first place, it is usually necessary to cast the concrete in heavy forms capable of supporting the very high tensile forces required in stressing the reinforcing steel. Even more troublesome than this is the fact that concrete shrinks when setting. It also has a surprisingly high coefiicient of plastic flow. Moreover, the steel is subject to creep under high tension, which is still another undesirable factor. In general, shrinkage, plastic flow and creep account for a loss of a very considerable portion of the prestressing force imposed on the steel. Manifestly, nothing can be done to recoup this loss because it occurs in major part after the concrete has set and become firmly bonded to the steel. Moreover, it is difiicult if not altogether unfeasible to measure the effective prestressing actually existing in the finished product, to say nothing of theimpossibility of varying this stressing either negatively or positively.

By using the new techniques and procedures of the present invention, I am enabled to poststress concrete in a very simple and highly effective manner. I have also developed a new method of erecting arched structures in such a way as to post stress concrete components thereof and simultaneously to transfer compression stresses in certain skeletal elements to the aforementioned concrete components.

Accordingly, it is a principal object of my invention to provide a novel arch structure utilizing post-stressed concrete members.

Another object is the use of unique methods and techniques to erect an arched structure.

A further object of the invention is to provide a beam construction making use of post-stressed, prefabricated concrete slabs.

Still another object of my invention is to so erect an arched structure that loads which would normally be carried by arched chords is transferred to a concrete slab covering in such a way as to post stress the slabs and increase their strength, as well as the stability and rigidity of the structure as a whole.

Yet another object of the invention is to post stress the composite elements near the springing line of an arched structure in such manner as to increase the stability and strength of other composite elements in the upper or crown portion of the arch.

Still another object is to provide a composite beam having a great depth in proportion to its span and having provision for post stressing the elements thereof in the order and by the amounts required to carry the design load most efiiciently and effectively.

Numerous other objects and advantages of my invention will become apparent from the following description of illustrative embodiments taken in connection with the drawings in which:

Figure 1 is an isometric view of one bay of an arched structure according to the invention taken from one end thereof and showing approximately one half of the span;

Figure 2 is a deflection diagram of the crown slabs taken in part along one end of the slabs and in part along a parallel plane midway between the slab ends;

Figure 3 is a fragmentary top plan view of one end of the crown slab showing details of the adjustable rigid ties between the slabs and the spanning arches;

Figure 4 is a top plan view of the composite spring line beam showing construction details and some of the stresses acting thereon during initial post stressing steps;

Figure 4a is a stress diagram of the composite beam after longitudinally grouting the slabs and tightening the tie bolts at both ends of the slabs.

Figure 4b is a stress diagram of the composite beam showing the forces acting after grouting the top ties and thereafter further tightening the lower bolts;

Figure 4c is a similar stress diagram showing the effects of the thrust load of the crown slabs on the spring line slabs;

Figure 4d is a stress diagram showing the algebraic sum of the stresses acting in Figures 4a, b, and c as they exist in the completed structure when made in accordance with the second or third techniques discussed hereinafter; and

Figure 5 is a view similar to Figure 4 and showing the various shear forces acting in the composite beam.

Fig. 6 is a sectional perspective view through the arch showing the embedded connector for the slab holding means The essential components of a preferred embodiment of my invention are illustrated in Figure 1, which shows one end half of the portion of an arched roof supported by a pair of arched chords 2t), 20. The ends of these chords are suitably supported against displacement by the usual foundations and the curvature of the rigid chords is chosen to conform with the funicular curve of the assembled roof structure. The chords are interconnected near their lower ends by a cross beam 21 and by a rib 24 parallel to but spaced above beam 21. The mating beams and ribs of adjacent bays of the arched span form continuations of these members in the structure as a whole. If desired, the junctions of beam 21 with chords 20, 20 may be supported in part by vertical columns in accordance with common practice.

The members so far described constitute the skeleton or frame of the structure. For reasons which will be explained below, the skeleton preferably has a design strength somewhat less than that required to support the dead load of the frame and the covering. It is even feasible to design the arches with only sufficient strength to support themselves. After the bolts are tightened and compressive stresses have been transmitted to the covering slabs the dead weight is thereby transmitted to the arch feet 23. Thereafter, portions of the arch 20 extending between the chords 21 on opposite sides of the structure could theoretically be removed or eliminated entirely for dead load conditions. This is because my specially designed covering has great strength and is employed to -supplement the skeleton in carrying the variousloadsr. .In

order to support the framing during erection and until the covering is conditioned to assist in performlng this function, the arches are supported initially by a series of ternporary columns29 and adjustable jacks 30. If desired column 31 and jack 32 may be placed beneath the mid 'arches 20,20.

Each of slabs 27 making up the composite panels or beams at the lower ends of the span have heavy steel plates, angle bars or the like 33 projecting therefrom, and

preferably from opposed corners, as appears more clearly in Figure 4. These projections form rigid ties which are embedded in the body of the slab'and suitably connected with a network of embedded reinforcing steel arranged to transmit horizontal shearing stresses between beam 21 and 'rib 24. The network of'reinforcing steel has not been illustrated since any one of a great variety of arrangements well known to reinforced concrete designers may be employed. The outer end of each of the ties has a hole to receive a heavy duty bolt 34 carried by a mating rigid tie 35 mounted on beam 21 and rib 24 as by welding, riveting or the like. The general details of these ties are illustrated more clearly in Figure 3. Slight spacing is left between ties 33 and 35 initially to provide for lateral movement of the slabs lengthwise of beam 21 and rib 24, 'as bolts 34 are tightened during erection of the structure.

Slabs 28 have similar ties which are connected by bolts to angle iron ties mounted along the top side of arches 20 by welding or the like. Accordingly, the same reference characters are used in describing them. The only distinction is that, as shown, ties 33 are located directly opposite one another near the upper lateral edge of slabs 28. However, they may be disposed near the lower edge of the slabs or, in fact, at any intermediate point across the ends provided the interior reinforcing network of the slabs is properly designed to distribute the stresses. The slabs 28 at or near the top of the roof crown need not be provided with ties for reasons which will be explained below.

In erecting the structure, one first erects the skeleton work in the customary manner while making use of the series of temporary columns 29 and 31 and jacks 30, 32 to aid in holding the arches in their predetermined design shape. Thereafter, slabs 27 and 28 are put in place in the arrangement illustrated and ties 33 and 35 are connected together by bolts 34. Sufficient space is left between the slabs to receive concrete grouting 36.

At this point it is possible to follow one of several techniques to post stress the slabs so as to place them under compression normal to their grouted joints to form a rigid, continuous, beam-like structure capable of supplementing the skeleton to a marked degree proportional to the magnitude of the compression forces acting across the slabs in carrying the load of the structure as a whole. According to one post stressing technique, this result'can be obtained by post stressing crown slabs 28. Another technique is to post stress slabs 27 at the opposite ends of the arch and utilize the resultant forces to post stress crown slabs 28. A third technique involves'combining the first two techniques.

Application of the first technique will now be discussed with the aid of Figure 2, which graphically represents conditions in the left half portion of the crown. It will be assumed that the skeleton has been erected, that the columns 29 and jacks 30 are in place, and that slab's' 28 have been assembled and grouted. The grouting is pref erably restricted to the mid portion of the longitudinal joints as indicated by lines 38 and 39 in Figures 1 and 3. Restriction of the grouting to this mid-area portion of the arch is helpful in explaining the'manner in which the deflection of the slabs is corrected by post-stressing. However, it will be understood that the same or similar corrective action is obtained even when the entire longitudinal joint .is grouted before'post-st ressing forreasons which will be explained below.

Line 40 in .Figure.2 represents. the. middle .planeof slabs 28 at a point overlying arches 20. The central portions of the slabs will deflect or sag downwardly between the arches due to their weight. Line 41 is similar to line 40 but is located in a vertical plane half way between and parallel to the arches. The positions of lines 40 and 41 are more clearly apparent from Figure 1.

The distance between lines 40 and 41 therefore represents the magnitude of the deflection of the slabs at the mid point between the arches. Note that this distance is greater at the top than at the lower end of the crown. This is due to the fact that the lower crown slabs are inclined to the horizontal and accordingly have a greater resistance to deflection than the horizontally positioned slabs at the top of the crown.

Bolts 34 in each slab are now tightened progressively. It is preferable to start fromthe lowermost slab and work upwardly along thearch.- In tightening the bolts it is desirable to maintain the loads on all bolts as nearly equal as possible. This may necessitate re-tightening of the bolts several times for the most satisfactory results. The grouted joints between the slabs do not separate as the tie boltsare tensione'd-if the bolts are tightened in a directionto maintain the joints constantly under compression, that is to say,-progressively from the opposite sides of the arch upward toward the crown.

Tightening of the bolts accomplishes several things. Thus, it takes up for the shrinkage occurring when the grouting sets. It further creates elastic compression in the slabs. The slack arisingfrom these dual sources is approximately equal for each slab. When this slack is taken up by the tightening of the bolts, the slabs are shifted along the arches toward the top of the crown. The movement of a particular slab depends upon its distance from the crown as indicated by arrows 42 which are longer at the lower ends of the crown to indicate graphically the greater sidewise movement of these slabs.

Bearing in mind that the pressure on each bolt is approximately the same, it will be apparent that the resultant compressive forces thereby transmitted from slab toslab increase directly as the number of bolts being tightened. Arrows 43 indicate the equal pressures applied to the slabs by the bolts, while arrows 44 represent the resultant compressive forces transmitted between the slabs. Note that Figure 2 indicates that only the bolts in the lower slabs have been tightened. Bolts and ties may be dispensed with for the slabs at the top of the crown, as indicated in Figure 1. The reason for this will be clear from Figure 2 which'shows that any compressive force introduced in the lower slabs is transmitted in substantially full strength to each slab thereabove.

One of the important features of the invention will now be explained, namely, the manner in which poststressing resulting from tightening the bolts as described, not only strengthens the slab arch, but actually substantially reduces the 'sag of the slabs between the arches. Reduction of-the sag further strengthens the slab arch.

Due to the omission of the grouting near the-ends of the slabs, it will be cvident that the tightening of the bolts tends to reduce. the' gap between the slab ends. Accordingly, the tightening pressures applied by the bolts sets up bending stresses crosswise of the slabs. These stresses are transmitted along the slabs to their mid por- ,tions where they are effective toplacethe grouting under increased compression toward theztop of the crown with the result that'tlie sag ofi the slabs is very substantially reduced In other words,nsag is .caused by dead load forces having major components acting downwardly toward the opposite lateral sides'of the arched span and these components :are opposed in whole or in part by the compressive forces acting laterally of the slab and toward. the. crownxof'thfe archas aresultof the tightening '0f1th6 tie bolts; Obviously;the=counteracting compressive forcesr-imposed-onthe slabs-from 'the'opposite sides of the .arch are'transmitted to'ithe arched ch'ordsby the tie members so as to place the chords under high tension lengthwise of the-chords. Itwill also be clear that the entire crown of the archis placed under high compressive stresses from its lower edges toward the top, causing the entire arch, as well'as the mid portion of the slabs, to be arched upwardly. The bolts are not tightened tothe extent that the uppermost slabs are elevated out of contact with the chords-20.---

a r Y The possibilities in this connection are somewhat startling because, as will be'readily perceived, post-stressing can actually make a slab arch self supporting. In fact, post-stressing can result in lifting the skeletal arches themselves upwardly. This upward movement of the arched roof, which of course is most pronounced at the top of the arch, has two principal causes. In the first place the upward arching of the slabs reduces or removes the dead load of the slabs on the arches. Secondly, the post-stressing applied progressively from the lower ends of the arched slabs upwardly towards their crown forces the crown upwardly.

This action is graphically represented in Figure 2. It will be remembered that line 41 passes through the mid plane position or plane of maximum sag, of the slabs before post-stressing. Line 41 represents the new position of the sag after post-stressing. As will be clear from the foregoing discussion, line 41 may have various positions and curvatures depending on the degree of poststressing. Its crown could actually lie above line 40, particularly if bolts 34 are tightened sufficiently to remove sags in main arches 20.

The upward deflection increases progressively from the lowermost bolt upwardly toward the crown. Manifestly, post stressed slabs will adjust themselves to the funicular curve. As a result of this, any deflection of arches 20 which may have occurred between supporting columns 25;) ie eliminated, further strengthening the structure as a w o e.

While the foregoing explanation has been given for a structure in which the grouting is omitted laterally beyond lines 38 and 39, it is to be understood that very substantial correction of the slab sag is obtainable with grouting extending the full length of the slabs. This is due in part to the elastic properties of both the grouting and the slabs as well as the fact that only infinitestirnal increments of movement in the plane of the slabs is required to give a very pronounced corrective movement to the sagged area, as can be readily demonstrated mathematically.

As soon as this corrective post-stressing procedure has been completed, grouting is supplied to the unfilled spaces between the slabs, as well as at the ends of the slabs so as to enclose the rigid tie and bolt assemblies.

In an actual roof structure therewill usually be a number of spans at either side of the one discussed. After each of these has been post-stressed, following the procedure described, ungrouted spaces are filled in. Jacks 30 can then be lowered and columns 29 removed. If it isdesired to use the jacks and columns in erecting successive spans, it is only important to fill in the grouting throughout the full length of the slabs and in the space between the ends of the post-stressed slabs overlying those arches from which the temporary columns are to be removed. Employment of the second technique, which will now be described, is dependent largely upon forming the spring line slabs 27 into a stiff beam. The manner in which this is done will best be understood by reference to Figures 4 and 5. Slabs 27 are similar to slabs 28 and differ essentially in having a somewhat different arrangement of the embedded reinforcing steel. These slabs have their opposite ends supported on beam 21 and rib 24. Of course, rib 24 also underlies the edge of the lowermost crown slab 28. Rigid ties 33 on slabs 27 are connected to mating ties 35 by bolts 34, not shown.

The next step is to grout the longitudinal edges of the slabs After the grout has set, bolts 34 are tightened at both ends of the slabs sufficiently to take up the shrinkage of the grout and place all slabs under an initial compression. This action also places both beam 21 and rib 24 under an initial tension. The action obtained is generally similar to that accomplished by tightening the bolts on slabs 28, except that there is no arching movement of the slabs since they all lie in the same plane. By properly tightening the bolts at both ends of the slabs in increments, all portions of the slabs are placed under compression, while beam 21 and rib 24 are placed under tension as is graphically illustrated in Figure 4a. As in crown slabs 28, the compressive action increases progressively towards the center of the panel. Preferably, all of the bolts are placed under the same tension, but the effects on the slabs and on the beams are cumulative toward the center.

' The stress diagrams of Figures 4a to 4d depict the general nature and distributionof stresses in beam 21, slabs 27 and rib 24 under differing conditions. In each of the figures the arrows pointing to the right represent the relative magnitude and line of action of the resultant compressive forces, while the arrows pointing to the left represent the relative magnitude and line of action of the resultant tensile forces. The forces acting lengthwise of beam 21 and rib 24 are represented in the narrow bands at the lower and upper ends, respectively, of the diagrams, while the intervening triangular or rectangular portions of the diagrams show the distribution of forces acting across the width of slabs 27 from one end to the other thereof.

At this point, it is appropriate to note that the rigid composite beam consisting of beam 21, rib 24 and slabs 27 just described would ordinarily extend the full length of the span and would therefore form a part of a continuous beam structure. In continuous beams it is desirable to concentrate the compressive forces in the areas of the anticipated maximum tensile stresses. This end is easily obtainable in my structure by so arranging the rigid ties 33 and 35 that tightening of bolts 34 achieves the stress pattern best adapted to counter-act the tension expected from dead and live loads.

Let us now return to the manner in which post-stressing of slabs 27 can be utilized to post-stress crown slab 28.

The first step is to grout the space between the upper ends of the slabs 27 and the lowermost of slabs 28. Thereafter, bolts 34 at the lower end of slabs 27 are tightened progressively to force the lower ends of the slabs together. As a consequence, the mid portion of rib 24 is bowed upwardly, as indicated by line 45 in Figure 4. Another result is that the lower areas of slabs 27 are placed under progressively increasing compressive forces towards the center of the panel, as indicated by arrows 46 and by the stress diagram in Figure 4b. The changed conditions brought about by tightening the bolts is best understood by comparing the stress diagrams in Figures 4a and 4]).

From what has just been said, it will be evident that the composite beam formed by beam 21, rib 24 and slabs 27 has not only been made very rigid, but it has been bowed upwardly in its own plane to apply a maximum compressive force to the longitudinal central areas of crown slabs 23. This action serves in a most effective manner to post stress the crown slabs and to bring about the several beneficial results described above in connection with the first technique.

Figure 46 represents the stress distribution resulting from the forces transmitted to the composite beam referred to in the preceding paragraph by the arch pressure acting in the crown slabs. This pressure tends to put the lower portion of the composite beam in tension. Since the grouted joints between the longitudinal edges of slabs 27 are not designed to take tensile loads, the stress diagram for the lower halves of slabs 27 is represented in Figure 4c by dotted lines. This over-all, true stress distribution in the complete arch structure is represented in Figure 4d, which represents the algebraic sum of the stresses shown in Figures 4 2, b and c.

Since the vertical joints between slabs 27 are in compression, the designer can safely count upon them for transmission of vertical shears, as the joints now have high resistance to relative vertical shifting of the slabs. This condition is illustrated in Figure 5. It may therefore be stated that one of the important functions of the rigid ties on slabs 27 is to transmit the horizontal shears between the rigid beams or ribs extending across the opposite ends of the slabs. These horizontal stresses vary in magnitude and correspond to the increment of the stresses acting in the beams between adjacent ties. Figure 5 illustrates how the vertical shears acting at the joints balance the horizontal shears acting through the ties and along the beams to maintain the components of the composite panel formed by slabs 27 in equilibrium.

it is highly significant that in the completed structure, marginal beam 21 is in high tension and rib 24 is under slight tension as graphically represented by the lengths of the arrows at bottom and top, respectively, in Figure 4d. Moreover, and of particular importance is the fact that the entire area of slabs 27 is under substantially uniformly distributed compression. it will be appreciated that the crown slabs are also acting in compression throughout their extent and that both the spring line and the crown slabs are either self supporting by arching action, or nearly so, just as was the case when applying the first technique.

The third technique, as stated initially, involves combining the first and second post-stressing procedures. As will be obvious from the discussion of the first two pro- .cedures, it is feasible to proceed in a variety ofways in carrying out the thirdtechnique. However, the preferred method is to apply the first technique of post stressing to the crown slabs and to thereafter post-stress the spring line slabs. As the latterare stressed, upward pressure is applied to the central area of the crown slabs to supplement and increase the post-stressing and-corrective results already obtained in post-stressing slabs 28. The advantage particularly in the lower areas of the crown. This is because the lower crown slabs are placed under high bending stresseswhen used alone in accordance with the first technique to develop the arch load. This condition is very materially relieved by combining the first and second techniques and utilizing the upward arching action available by post-stressing the spring line slabs to absorb part of the slabs by following the second technique, at least through the step illustratedin Figure 4a. When this has been done, the grouting is applied to the joint between spring line and crown slabs. When this grouting has set, the jacks can be removedsince the grouting will thereafter maintain the post stressing obtained initially by the use of the jacks.

Still other alternate procedures can be adopted for poststressing the slabs to obtain the new and beneficial results contemplated and coming within the scope of this invention. However, it is not felt that further elaboration with respect to this will serve any useful purpose, since these procedures fall within the skill of the art once the underlying principles of the invention previously discussed are understood.

In executing any one of the above procedures, it will be understood that the temporary columns 29 are removed as soon as the final grouting has been applied and set. Upon removal of the columns, the load previously carried thereby immediately causes the arch to tend to settle. This sets up counter-acting stresses in both the arches and the covering slabs, additionally post stressing both assemblies, and adding to the strength, rigidity and stability of the structure to a very marked degree. The principal reason for this is that substantially all parts of the structure are already under considerable compressive forces, all shrinkage and most of the elastic compressions having been absorbed in the post stressing previously applied. Consequently, any tendency of the arch to settle, however small, will be greatly resisted by the new and additional compressive forces set up in the slabs. These latter forces are in reality additional post-stressing forces.

of course is, that much greater strength can be developed,

The last fact stated above leads naturally to the conclusion that post stressing of an arch structure can be effectively accomplished without the use of rigid ties on any of the crown slabs provided a rigid beam is employed at the spring line of the arch. A rigid beam of the required strength is most conveniently obtained by utilizing the composite beam panel formed principally of slabs 27, and described elsewhere in this specification. The crown slabs are erected in the manner shown and the grouting is filled in. After it has set, the temporary columns are removed.

The arch will settle to a somewhat greater degree than was the case when the crown slabs were post-stressed by the use of the rigid ties. This greater settling tends to decrease the length of the arch thereby automatically creating high, post-stressing compressive forces in the slab cover. crease the strength, stability and rigidity of the covering, but-it will also reduce the sag or the crown slabs in the same general way and forthe'reasons explained fully in connection with the discussion of the first technique.

Not only will the post stressing so obtained in- 'Obviously the post-stressing obtainable in the covering slabs when employing the last described method can be Still 'an'otherirnethod of post-stressing fallingwithin this inventive concept involves the use of an expanding..type of grouting. Such, a grouting. can be made .by adding a powdered metal and oxidizing chemicals to the grout mix. As this grouting sets and expands it automatically sets up compressive stresses acting normal to the'slab edges to post stress the slab covering.

Fromthe foregoing it will be evident that the present invention makes it possible to construct very wide spans from simple, light-weight prefabricated components to provide a structure having unusual and highly advantageous characteristics.

It will be understood that the principles of the invention can be carried out in various ways and by means of structures other than those specifically described above, all of which are to be considered a coming within the scope of the invention. 7

I claim:

1. That method of erecting a wide spanned structure which comprises erecting spaced arches parallel to one another, placing temporary supports below said arches capable of supporting a major portion of the dead load of the structure during the erection thereof, covering the crown from one side thereof to the other with prefabricated slabs laid edge to edge and spanning the distance between adjacent arches, grouting the joints between the slabs and thereafter post stressing said slabs by compressing the same throughout their lengths and from theopposite sides of said arch to provide a high strength arch having great stability and rigidity and in which a major portion of the dead load thereof is carried directly by said covering.

2. The method of erecting a wide spanned structure as defined in claim 1 wherein said slabs are post stressed by forcing the opposite ends thereof upwardly toward the top of the crown from the opposite sides thereof and securing the slabs rigidly to said arches in said stressed condition.

3. The method of erecting a wide spanned structure as defined in claim 1 wherein said slabs are post stressed by removing said temporary supports and allowing said structure to settle slightly with the result that permanent high compressive forces are introduced into said covering from one side thereof to the other.

4. That method of erecting a wide spanned structure which comprises, erecting spaced arches parallel to one another, temporarily supporting said arches at spaced points along their length, placing a covering crosswise of said arches consisting of prefabricated concrete elements arranged edge to edge, filling those portions of the crevices between adjacent elements extending from the center thereof to points spaced from the ends of the elements with grouting, and thereafter post stressing said elements by applying pressure to the opposite ends of said elements crosswise thereof from the opposite sides of said structure upwardly towards the crown thereof to compress said elements together along the longitudinal edges thereof whereby pressures are set up in said elements to increase the load carrying ability thereof and to apply sag corrective forces to said elements lengthwise of said arched structure.

5. The method defined in claim 4 including the steps of locking said post stressed elements to spaced points alongsaid arches and filling the unfilled portion ofthe open crevices and the areas at the ends of the elements with grouting.

6. The method of erecting an arch skeleton having rigid marginal side beams which comprises, erecting -a plurality of spanning arches in spaced apart vertical planes, connecting each of the lower ends of said arches together with a pair of spaced rigid members, covering the space between said rigid members with prefabricated slabs extending crosswise of each of said pairs of rigid members in edge to edge relation, adjustably securing the upper and-lower ends of said. slabs tosaid rigid members so that said slabs can be compressed toward one another from the opposite side edges thereof, filling the crevices between adjacent slabs with grouting and thereafter post stressing said slabs by complurality of spanning arches in spaced vertical planes, connecting the lower ends of said arches together by spaced, rigid members, covering the space between said members with prefabricated concrete slabs extending crosswise thereof in edge to edge relation, adjustably and rigidly securing the upper and lower ends of said slabs to said members so that said slabs can be compressed toward one another from the opposite side edges thereof, filling the crevices between adjacent slabs with grouting, grouting the area at the upper ends of said slabs, and thereafter compressing the lower ends of said slabs together and locking the same while so compressed to said lower rigid member to form wide area, composite, rigid side beams at either end of said arch skeleton which are deflected upwardly toward the crown of said arch whereby, said deflection can be utilized to post stress a composite crown covering for said arch skeleton.

8. That method of erecting a wide spanned structure which comprises, erecting a skeleton including a pair of spanning arches in spaced vertical planes, and. a pair of spaced, horizontal rigid members connecting the lower ends of said arches, utilizing said skeleton to form the chords of two composite rigid beams with flat surfaces lying in the surface of the arch structure at either side thereof, erecting a third composite, rigid beam across the crown of said structure, said third beam having an arched surface extending crosswise of said spanning arches between the upper edges of said first mentioned composite beams at either side of said structure, and forming each of said composite beams by placing prefabricated slabs edge to edge crosswise of the supporting skeleton members, grouting the crevices between the adjacent edges of the slabs and thereafter compressing the slabs together from the ends of each of said beams and locking the compressed slabs to said skeleton to provide an arched structure having a cov ering under compressive stresses substantially throughout the entire extent of the same and acting between the upper and lower surfaces of said slabs.

9. That method of making a self-supporting arched beam which comprises, arranging a row of elongated prefabricated concrete slabs in edge to edge relation across the crown of the arch, similarly arranging rows of corresponding spring line slabs at right angles to said row of crown slabs across the opposite ends of said row, grouting the crevices at the longitudinal joints between slabs, compressing the slabs together from the opposite ends of each row thereof, and locking the slabs in this compressed condition to form a composite arched beam of high strength, stability and rigidity.

10. That method of making a self-supporting arched beam defined in claim 9 including the step of compressing the lower portions of said rows of spring line slabs more than the upper portions thereof whereby said rows of spring line slabs are arched upwardly in the plane thereof to resist the thrust load imposed by the row of crown slabs resting against the upper edges of said spring line slabs.

11. That method of constructing a wide spanned structure without permanent supporting columns as defined in claim 9 wherein a plurality of said self supporting arched beams are positioned side by side and permanently locked to one another to form an arched structure of any desired length having high strength, stability and rigidity throughout and in which, parts of said slab covering are under compressive stresses acting between the upper and lower surfaces of said slabs.

12. In combination, a wide spanned structure comprising, a plurality of arched members in spaced apart vertical planes parallel to one another, rigid beam means interconnecting the adjacent ends of said arches, an arched row of prefabricated slabs overlying and spanning the distance between consecutive arches and extending across the crown thereof between said rigid beam means, the crevices to either side of said slabs being filled with grouting, said structure including means holding said slabs under high compressive stresses acting crosswise of said grouted crevices and upwardly toward the top of the crown whereby substantial portions of the dead load of said slabs are supported by the slabs themselves and whereby the strength, stability and rigidity of said structure is greatly increased.

13. A wide spanned arched structure as defined in claim 12 wherein said means holding said slabs under compression comprises adjustable rigid ties between the ends of certain of said slabs and the adjacent portions of said arched members, said ties being adjustable to move the slabs upwardly along the arched members from the opposite sides thereof to post stress said slabs.

14. A wide spanned arched structure as definedin claim 12 wherein each said rigid beam means comprises a pair of spaced beams, concrete slabs spanning said spaced beams and covering the space between adjacent arched members grouting in the crevices between the last mentioned adjacent slabs and adjustable means interconnecting the last mentioned slabs: and said spaced beams for moving the last mentioned slabs towards one another to place the same under compression whereby said spaced beams and said slabs form a composite beam having great strength, rigidity and stability.

15. That method of erecting a wide spanned structure which comprises, erecting spaced arches parallel to one another, placing temporary supports below said arches capable of supporting a major portion of the dead load of the structure during the erection thereof, connecting the lower end portions of said arches with spaced apart rigid members, covering said rigid members at either ends of said arches with a row of spring line slabs comprising elongated slab members having their longer edges extending in the same direction as the adjacent portions of said arches, covering the intervening crown portion of said arches with elongated slabs extending at right angles to the length of said spring line slabs and spanning the distance between adjacent arches, grouting the joints between all of said slabs to form a rigid arched covering for said arches having a continuous weather tight surface, and removing said temporary supports.

16. That method of erecting a wide spanned structure as defined in claim 15 which includes rigidly connecting the ends of said spring line slabs to the adjacent portions of said rigid members and rigidly connecting the ends of said crown slabs to the adjacent portions of said arches.

17. That method of erecting a wide spanned structure as defined in claim 15 which includes rigidly connecting the ends of said crown slabs to said arches and simultaneously applying compressive forces to said slabs from the opposite sides of said crown upwardly toward the apex thereof.

18. That method of erecting a wide spanned structure as defined in claim 17 which includes rigidly connecting the lower ends of said spring line slabs to the adjacent portions of said rigid supporting members and simultaneously compressing said slabs together from the outermost slabs toward the center slab in each spring line row thereof thereby causing the upper end of each spring line row of slabs to be deflected upwardly to apply compressive stresses to the central portion of said crown slabs and from the opposite sides of the slabs in the crown of said structure.

19. That method of erecting a self-supporting widespanned, arched structure which comprises supporting a plurality of arched members on temporary columns, interconnecting adjacent arch members with pairs of spaced beams adjacent the spring line thereof, the crown por tions of said arch being temporarily supported at a higher elevation than they will occupy after the temporary supporting columns are removed, said arched members be: ing arranged in spaced parallel relation, covering the lower spring line portions of said arched structure with pre-fabricated slabs rigidly inter-locking said slabs with said spaced beams to provide a relatively thin, wide area beam at either side of said structure having great rigidity for supporting the compressive loads imposed thereon by the crown of said structure, covering the crown por tion of said arched structure intervening between the upper edges of said rigid wide area beams with pre-fabricated crown slab members laid edge to edge cross-wise between said arch members, securing the ends of said crown slab members to said arch members, grouting the space between the edges of said crown slab members and then removing said temporary columns and allowing said structure to settle whereby the entire covering thereof is placed under high compressive stresses acting between the upper and lower surfaces of said covering.

20. That method of erecting a self-supporting, widespanned, arched structure as defined in claim 19 which includes securing said pre-fabricated crown slab members to said arch members so as to leave a narrow space between the adjacent edges of said slab membersgantl filling said .narrow spaces,-as Wellas the space between said members at the top of said crown, with grouting whereby compressive stresses imposed on said-arched members upon the removal of said temporary columns are distributed to said crown slab members.

21. That method of erecting a self-supporting, wide spanned, arched structure which comprises mounting the opposite ends of spaced arched chords in vertical parallel planes, supporting the ends of said chords rigidly against thrust loads acting downwardly through said chords, supporting intermediate portions of said chords on temporary columns, interconnecting the opposite lower vend portions of said chords by relatively wide but thin rigid beams placed under compression from the ends thereof connected to said chords whereby the same are ,capable of absorbing thrust loads imposed by the covering applied across the crown of said structure, covering the crown of said structure intervening between the vupper edges of said rigid beams at the opposite ends of said chords with thin and relatively wide pre-fabricated members laid edge to edge crosswise of said chords in slightly spaced relation to one another, filling the central pottion of the space between said members with material to hold the members spaced apart, rigidly connecting at least certain corners of said members to the adjacent portions of said chords so as to place said covering members under compressive stresses acting between the upper and lower surfaces thereof across said crown, removing said temporary columns and then grouting the unfilled spaces between the edges of said covering members to provide a continuous, thin, rigid covering from one side of said structure to the other.

References Cited in the file of this patent UNITED STATES PATENTS 

